1. Field of the Invention
The invention relates to optical systems, for example projection objectives or illumination systems, of microlithographic projection exposure apparatuses as are used for the production of microstructured components. The invention also relates to a method for improving imaging properties of such optical systems.
2. Description of Related Art
Microlithographic projection exposure apparatuses, which are used for the production of large-scale integrated electrical circuits and other microstructured components, contain an illumination system which is used to generate a projection light beam. The projection light beam is directed at a mask, which contains minute structures to be imaged and is arranged in an object plane of a projection objective. The projection objective forms a reduced image of the structures contained in the mask on a photosensitive layer, which is located in an image plane of the projection objective and may, for example, be applied on a wafer.
Owing to the small size of the structures to be imaged, very stringent requirements are placed on the imaging properties of the projection objective, and increasingly of the illumination system. Imaging errors of these optical systems must therefore be reduced to a tolerable level by suitable measures.
In this context, it has been known for a long time to change the position of individual optical elements inside the relevant optical system with the aid of manipulators. Such position changes, however, can only correct a few imaging errors retrospectively. Further imaging errors can be corrected by changing the shape of optical elements, or more precisely their reflective or refractive surfaces.
In connection with lenses, for example, it is known from U.S. Pat. No. 6,388,823 B1 assigned to the applicant to bend a lens without significantly changing its thickness. To this end, the lens is engaged circumferentially by a plurality of actuators which generate the intended bending moments in the lens.
EP 1 376 192 A2 discloses a projection objective of a microlithographic projection exposure apparatus, which is constructed exclusively using mirrors. Two of the mirrors can be deformed so that their reflecting surfaces respectively change in shape. Various configurations of actuators which can provide an intended deformation of the mirror surface are also described.
Optical elements with deliberately deformable surfaces, which are often also referred to as active or adaptive optical elements, are also suitable for correcting those imaging errors which vary as a function of time. For example, there are imaging errors which are due to changes in refractive index and/or shape, which are in turn a result of heat produced in the optical elements by the projection light. The energetic projection light can furthermore lead to irreversible material modifications at those places on the optical elements which are exposed to the projection light. It is also known that the shape of optical elements can even change because of settling and relaxation effects when they are not exposed to projection light.
When time-variable imaging errors, attributable to the aforementioned or similar causes, are intended to be corrected by changing the shape of individual surfaces during operation of the projection exposure apparatus, then the corrective measures must be designed so that they can be implemented as much as possible in short exposure pauses. Corrective measures which require longer down times of the apparatus reduce the throughput and therefore compromise their economic viability.
In connection with projection objectives, it has therefore been proposed to analyze the imaging properties of the projection objective during exposure. It is known, for example from US Patent Application 2003/0002023 A1 assigned to the applicant, to couple a measuring light beam into the projection objective so that it lies outside the actual projection light beam after it emerges from the projection objective. The emergent measuring light beam is analyzed with the aid of a wavefront detector, so that it is possible to infer the imaging properties at least of that part of the projection objective through which the measuring light beam has passed. On the basis of these measurements, corrective measures can then be determined which inter alia may comprise changing the shape of adaptive lenses or adaptive mirrors. A similar method is also known from EP 1 376 192 A2, which was already mentioned above.
Such known measuring methods, however, can quantitatively register only some specific imaging errors. The causes explained above may, however, also induce time-variable imaging errors which cannot be analyzed during projection operation by the known method.